Plasma assisted molecular beam epitaxy (MBE) is the deposition of a III-V semiconductor by MBE where the group V source (for example nitrogen) is generated by a plasma cell.
The (Al,Ga,In)N material system includes materials having the general formula AlxGayIn1−x−yN where 0≦x≦1 and 0≦y≦1. Herein, a member of the (Al,Ga,In)N material system that has non-zero mole fractions of aluminium, gallium and indium will be referred to as AlGaInN, a member that has a zero aluminium mole fraction but has non-zero mole fractions of gallium and indium will be referred to as InGaN, a member that has a zero indium mole fraction but has non-zero mole fractions of gallium and aluminium will be referred to as AlGaN, and so on.
The (AlGaIn)N family of materials can crystallise in either the zincblende (cubic) or wurtzite (hexagonal) crystal structure. The wurtzite crystal structure is more common and it is this crystal phase which is currently used in commercial LEDs and LDs made from (AlGaIn)N. However, the zincblende crystal structure of (AlGaIn)N offers several potential advantages over the wurtzite crystal for LEDs and LDs; these are:                i) Zincblende (AlGaIn)N is a non-polar semiconductor material, i.e. the cation (Ga, Al, In) and anion (N) atom planes are not stacked sequentially. Non-polar semiconductor materials do not suffer from strong internal electric fields which can reduce the optical efficiency of the material.        ii) The electronic band structure of zincblende (AlGaIn)N predicts improved optical gain and higher p-type conductivity of the material compared to the wurtzite phase.        iii) The narrower band-gap of zincblende InGaN compared to wurtzite InGaN means less indium is required to achieve a particular optical emission wavelength. Lower indium means less defect inducing strain in the crystal.        
To realise optoelectronic devices made from zincblende (AlGaIn)N, high quality zincblende (AlGaIn)N substrates are required onto which the devices are epitaxially grown by molecular beam epitaxy (MBE) or metal organic vapour phase epitaxy (MOVPE). However, high quality zincblende (AlGaIn)N substrates are difficult to achieve and are not commercially available at present.
Yang et al. have reported, in J. Appl. Phys. 83 (1998) p 3800, that zincblende GaN can be epitaxially grown on Silicon (100) using a GaAs buffer layer between the silicon and GaN to prevent the formation of amorphous SiN. However, the method of Yang et al. is undesirable as it did not produce zincblende GaN with suitable high optical quality. This is likely to be due to poor crystal quality through the use of plasma assisted MBE, which is a low temperature method, for the final GaN layer and not using arsenic or phosphorus mediated GaN epitaxy for initial growth on the GaAs layer.
Nishimura et al., Optical Mat. 19, p 223 (2002) have reported on cubic GaN grown on silicon using a boron phosphide interlayer. A table showing materials which can be grown on Si is also given; AlP, InP and GaP are in the table.
U.S. Pat. No. 6,194,744B1 discloses a method of forming zincblende (AlGaIn)N on a silicon substrate using a layer of boron phosphide between the (AlGaIn)N and silicon. This method is undesirable for zincblende (AlGaIn)N because of the difficulties associated with depositing boron phosphide, such as precursor toxicity. Also this deposition technique is limited to a vapour phase method such as MOVPE.
Zhao et al. report, in Appl. Phys. Lett. 74(21) 1999 p 3182, the use of phosphorus mediated MBE to grow zincblende GaN on a wurtzite GaN substrate. The use of a silicon substrate or a III-As/P interlayer is not mentioned by Zhao et al. Also, the method of Zhao et al. resulted in poor crystal quality and optical properties were not reported. Ammonia was not used for any of the MBE growth.
Cheng et al. discuss, in Appl. Phys. Lett. 66(12) 1995 p 1509, the use of arsenic to control the crystal phase of GaN grown on GaAs and GaP substrates by MBE. Cheng et al. make no mention of using silicon substrates or interlayers. In addition, ammonia was not used for any of the MBE growth. Moreover, PCT Publication WO2006/120401A1 describes the method of forming a bulk, free-standing zincblende III-N substrate by growing III-N material on a zincblende III-V substrate using MBE and then removing the substrate.
In view of the aforementioned shortcomings associated with conventional structures and techniques, the present invention provides a high quality zincblende (AlGaIn)N structure suitable as a substrate for zincblende (AlGaIn)N optoelectronic and electronic devices and a method of forming such a structure.